It is common for HIV infected persons to regain some weight as a result of improved antiretroviral therapies. This weight gain is however often modest, incomplete, and may be due primarily to increases in body fat. Failure to regain weight or lean tissue may be due in part to inadequate protein intake. ACTG 392 is a phase II, 12 week randomized, double-blind, study of HIV positive subjects who have lost at least 3% of their body weight and who now have stable weight loss. Subjects will be randomized to take either an oral nutritional supplement containing increased amounts of whey protein rich in cysteine and glutamine or an identical tasting supplement without added whey protein. Supplements will be taken mid-morning and mid-afternoon between meals. The study will determine if the whey protein supplement will result in better repletion and maintenance of lean tissue than an isocaloric supplement (without whey protein or amino acid). The impact of therapy on antioxidant status, energy and protein intake, body composition, and quality of life will also be examined.